The present invention relates to seals and in particular to mechanical face seals for gases, in which grooves are provided in one of the seal faces to establish a cushion of gas between the seal faces, when one seal face rotates relative to the other. While particularly suitable for use in sealing gases, seals of this type may also be used for other fluids.
With seals of this type, the pressure of gas between the seal faces and consequently the film thickness, is a function of the relative speed of rotation of the faces and the design of the grooves.
It is desirable that at slow speeds sufficient pressure will be developed between the faces to provide lift-off in order to minimise damage to the seal faces on start-up and shut-down. Furthermore, when the film thickness is small, particularly at low speeds, a film of high stiffness is required to prevent contact in the event of an instability during the running of the seal faces.
At higher speeds, the film thickness must be sufficiently large to make the seal insensitive to minor variations in counterface surface finish and to cope with swash of the seal faces and surface waviness. The running gap must also be stable to give a predictable controlled leakage and to maintain a constant flow of cooling gas across the whole face in order to ensure an even temperature distribution and avoid hot spots which would cause distortion of the seal faces.
Hitherto, these conflicting requirements have been accommodated by compromise in the design of the grooves.